Gastrointestinal polyps are being identified more frequently today because of increased awareness, screening and improved diagnostic tools. The entire gastrointestinal tract is at risk for polyp development but the adult colon and rectum account for the majority of polyps.

• Colorectal cancer is the fourth most common form of cancer in the US and has the second highest cancer mortality
  
• In 2002, 148,000 new cases of colorectal cancer were anticipated with more than 56,000 deaths
  
• There is a 6% life time risk of colorectal cancer. The incidence is higher in men, especially African American. Only about 35% of cases are diagnosed at an early stage
  
• The transition from benign (noncancerous) adenoma to carcinoma (cancerous polyp) has been established. A series of genetic mutations involving several genes is necessary for malignant transformation over an extended period (probably years with 95% of colorectal malignancies arising from adenomas)
  
• Gastrointestinal (GI) malignancy is unusual in childhood
  

Though any part of the GI tract may develop a polyp, the emphasis here is on colon polyps

Anatomy and Physiology

• The large bowel absorbs 90% of the water content of the digested food it receives from the small intestine. It also propels the residue towards the rectum, where it is stored and expelled with a bowel movement
  
• The colon is divided into four segments and two flexures (Figure 1):
  
1. Cecum is the first portion of the large bowel and is joined to the small bowel. The appendix lies at the lowest portion of the cecum (See Appendectomy)
   
2. Ascending colon extends upwards from the cecum to the hepatic flexure near the liver
   
3. Transverse colon extends across the upper abdomen to the splenic flexure
   
4. Descending colon extends from the splenic flexure downwards to the start of the pelvis
   
5. Sigmoid colon extends from the descending colon to the rectum
   
• The wall of the colon is composed of four layers (Figure 2):
  
  1. Mucosa - The epithelial (single layer of cells) lining is flat and regenerates itself every 3-8 days. Small glands lie beneath the surface.
     
  2. Submucosa - The area between the mucosa and circular muscle layer and is separated from the mucosa by a thin layer of muscle, the muscularis mucosa
     
  3. Muscularis propria - The inner circular and outer longitudinal muscle layers
     
  4. Serosa - The outer single cell thick covering of the bowel. Similar to the peritoneum, the layer of cells that lines the abdomen
     
• The colon does not have lymphatic channels in the submucosa between the mucosa and muscle. This is important because tumors invading into this area do not have lymphatic channels through which to metastasize (spread)

Figure 1 - Anatomy of the colon.	Figure 2 - Cross-section through the wall of the colon (see text). © C. Hogan